Colloids electrosteric stabilization

Capek I. Sterically and electrosterically stabilized emulsion polymerization. Kinetics and preparation. Adv Colloid Interface Sci 2002 99 77-162. 

The role of polymers on colloid stability is considerably more complicated than electrostatic stability due to low molecular weight electrolytes considered in Chapter 11. First, if the added polymer moieties are polyelectrolytes, then we clearly have a combination of electrostatic effects as well as effects that arise solely from the polymeric nature of the additive this combined effect is referred to as electrosteric stabilization. Even in the case of nonionic... 

Ottewill, R.H. and Satgurunathan, R. (1995) Nonionic lattices in aqueous media. 4. Preparation and characterisation of electrosterically stabilized particles. Colloid Polym. Sci., 273, 379-86. 

With polymers that have ionizable groups, adsorption of a polymer will alter the charge of the surface altering the electrostatic interaction energy and also provide steric protection for the colloid, because the ionized groups will give better than theta conditions for the poisoner in an aqueous solution. This type of polymer stabilization is called electrosteric stabilization because both the electrostatic and the steric play a role in stabilization. The equations for this total interaction are simply the sum of electrostatic and steric terms as well as the van der Waals attraction. 

Biggs, S. and Healy, T.W, Electrosteric stabilization of colloidal zirconia with low molecular weight polyacrylic acid, J. Chem. Soc. Faraday Trans., 90, 3415, 1994. 

Polymer 42 (R=Me) is a powerful stabilizer in emulsion polymerization of ST. The particle size increases with increasing length of the hydrophobic block. Due to very efficient electrosteric stabilization the resulting dispersions have outstanding colloid stability even at high salt concentration [64, 75],... 

Pettersson, A. et al., Electrosteric stabilization of Aljti, ZrOj, and SY-ZrOj suspensions Effect of dissociation and type of polyelectrolyte, 7. Colloid Interf. Sci., 228, 73, 2000. 

The major route to colloidal (effectively water soluble) PAn has been through the chemical oxidation (S2082-) of the monomer in the presence of polymeric steric stabilizers and electrosteric stabilizers (polyelectrolytes), such as poly(vinyl alcohol), polyGV-vinyl pyrrolidone), polyethylene oxide), polystyrene sulfonate), dodecylben-zene sulfonate, and dextran sulfonate. It has been found that the stabilizer can act simultaneously as a dopant, imparting new functionality to the polymer or additional compatibility for the final application. 

Surfactants in Aqueous Solution A very important component that is usually present in the lyophobic colloids is the surfactant. These molecules are amphiphilic, that is, a part of the molecule is much more polar than the other part. On the basis of the nature of the polar groups in the surfactant molecule, they are classified as ionic (anionic or cationic) and nonionic. When ionic-type surfactants are adsorbed onto polymer particles, they provide stabilization by electrostatic repulsion between them and when the nonionic type are adsorbed instead the mode of stabilization is by steric repulsion. Electrosteric stabilization is provided by polyelectrolyte chains that give place to both modes of repulsion electrostatic and steric. 

In 1999, the group of Roucoux studied a new series of easily synthesized ionic surfactants that efficiently stabilize active suspensions of rhodium colloids in the hydrogenation of arenes in a biphasic Uquid-Uquid medium [34-36]. The synthesis of N,N-dimethyl-N-alkyl-N-(2-HydroxyEthyl)Ammonium salts which provide an electrosterical stabilization has been obtained by one step quaternarization of N,N-dimethylethanolamine with the appropriate functionaUzed alkanes or by ion exchange. These salts HEA-CnX bear an alkyl chain containing n= 12-18 carbon atoms and can be prepared with various counter-anions X such as Br, Cl, 1, CH3SO3, BF4 (Scheme 11.2). 

Since the beginning of colloids science, however it is also known that the agglomeration of colloids and dispersed particles can be prevented or controlled by stabilization [8]. The attractive interactions between the colloidal particles, caused by van-der-Waals forces, need to be compensated by repulsive interactions. The latter can be based either on electrostatic repulsion due to same-sign surface charges (electrostatic stabilization), or on repulsion via a polymer shell formed through adsorption of polymers to the particle surface (steric stabilization, in presence of polyelectrolytes termed electrosteric stabilization due to additional charged-induced repulsion) [9, 10]. The stabilization by control of the interaction forces between colloidal particles has been in the focus of extensive research efforts. Already... 

Electrosteric stabilization causes the colloidal particles to become resistant against both electrolytes (based on steric repulsion) and temperature (based on the electrostatic interaction). Such stabilization is frequently used industrially by the application of mixed stabilizers (one electrostatic and one steric). Likewise, polyelectrolyte chains - either adsorbed or covalently bound - impart an extraordinary stability against electrolytes [63, 64]. This effect may contribute to stability in emulsion polymerizations containing a small percentage of ionic monomers in the monomer mixture. 

Pettersson, A., Marino, G., Pursiheimo, A., Rosenholm, J.B., 2000. Electrosteric stabilization of AI2O3, Zr02, and 3Y-Z1O2 suspensions effect of dissociation and type of poly electrolyte. J. Colloid Interface Sci. 228, 73—81. 

In addition to electrostatic colloid stabilization generated by anionic surfactants, liquid dispersions are also made from nonionic surfactants. Stabilization of the emulsion is achieved by electrosteric stabilization or by pure steric stabilization (2,13). Polyoxyethylene dodecyl ethers, polyoxyethylene nonyl phenyl ethers, and polyoxyethylene nonyl phenol ethers are a few surfactants typically used in emulsion polymerization with nonionic surfactants (14-16). Non-ionic emulsion polymerizations are characterized by lower critical micelle concentration than their ionic counterparts. Thus, the emulsion particle sizes are generally much larger than in the ionic polymerizations. The mechanism of radical entry and exit in polymeric surfactant stabilizer systems are different than in anionic systems. With water-soluble initiators, the kinetics depends on initiator concentration. 

Various DHBCs have successfully been used as a stabilizer for the purely steric or combined steric/electrosteric stabilization of ceramic precursor particles like AI2O3 [308,309], BaTiOs [310-314] colloidal silica [315-320] alumina coated Ti02 [321], Ce02 [322], o -Fe203 (hematite) [281,323-325], and various pigments [327] in aqueous solution. The solid amoimt can be stabilized to extremely high concentrations (up to 80 wt % [327]) showing the stabilization capabilities of optimized DHBCs. 

Simple low molecular mass polyelectrolytes can electrosterically stabilize inorganic colloids. Besides this function, low molecular mass polyelectrolytes have been widely used as additives in the controlled growth of diverse inorganic materials [35,38]. The addition of polyelectrolytes with strong inhibition ability can stabilize the amorphous nanobuilding blocks in the early stage, and then stimulate a mesoscale transformation [10] or act as a material depot in a dissolution-recrystallization process. Time-resolved study of the scale inhibition efficiency of polycarboxylates has shown that amorphous precursor particles were formed in the initial stages [89] and were also observed in other cases. 

Ramakrishnan S., McDonald C.J., Carbeck J.D., Prudhomme R.K. Latex composite membranes Structure and properties of the discriminating layer. J. Mem. Sci. 2004 231 57-70 Romero-Cano M.S., Martin-Rodriguez A., Nieves F.J.D.L. Electrosteric stabilization of polymer colloids with different functionality. Langmuir 2001 17(11) 3505-3511 Rau D.C., Parsegian V.A. Direct Measurement of the intermolecular forces between counterion-condensed DNA double helices— Evidence for long-range attractive hydration forces. Biophys. J. 1992 61(1) 246-259... 

Electrostatic force depends on two parameters, Nemst potential and Zeta potential. Nemst potential has little effect on stabilization and is not practically assessable. Zeta potential is the potential difference between the electroneutral region and actual nanoparticle surface. The condition for stabiUly of nanoparticles is that the electrostatic repulsive forces should be higher than the attractive van der Waals forces. Hence, a higher value of zeta potential will result in a more colloidal stabihty of nanoparticles [65]. The different strategies to increase the stability of nanoparticles for biomedical apphcations are as follows. The first is electrostatic stabilization, in which a electric double layer around nanoparticles is formed. The second is steric stabilization in which a steric stabilizer is chemically attached or adsorbed on the nanoparticle surface. In the third approach, both the above methods are combined. Electrosteric stabilization has many advantages, such as redispersibility of nanoparticles, ability to support high concentration of nanoparticle suspensions, and electrolytic insensitivity [65-68]. 

---